new API: methods on Client and Server objects, instead of raw functions

rewrite README: add new API, add examples, remove low-level details
implement @ckarlof's API and README.md suggestions

* use "computeX" instead of "getX"
* Rename (external) "a"/"b" to "secret1"/"secret2" to avoid footguns.
* add an API summary
* comment out the unimplemented "checkM2" method
be consistent about _num/_buf type-suffixes for internal variable names
use a new-agnostic constructor for Client and Server
README.md: improve formatting of API listing, add table-of-contents
Merge pull request #21 from warner/new-api

new API: methods on Client and Server objects, instead of raw functions